Conventionally, a developing unit using two-component developer may have a developer carrying member, a supplying compartment, and an agitation compartment, for example.
Developer transported in the supplying compartment may be supplied to the developer carrying member, and agitatingly transported in the agitation compartment.
Such developer may be circulated between the supplying compartment and the agitation compartment, wherein developer may be transported in opposite directions in each compartment.
In such developing unit, the developer carrying member may use some developer and consume some toner at a developing area of the developer carrying member for a developing process, and then used developer may be recovered in the supplying compartment.
Accordingly, such used developer, which consumed toner, may be mixed with un-used developer transported in the supplying compartment. Such un-used developer may mean developer transported in the supplying compartment but not be carried up to the developer carrying member.
The supplying compartment and the developer carrying member may have substantially identical lengths and be disposed parallel to each other. In other words, the supplying compartment may extend along an axial length of the developer carrying member.
Such supplying compartment may have an upstream side and a downstream side defined in terms of a direction of transport of developer in the supplying compartment.
In such supplying compartment, an amount of used developer, which passes over the developing area on the developer carrying member and is recovered in the supplying compartment, may increase toward the downstream side of the transport direction in the supplying compartment.
Accordingly, toner concentration in the developer, transported in the supplying compartment, may decrease toward the downstream side of the transport direction in the supplying compartment because toner was consumed at the developing area for the image developing process as described above.
Such decrease of toner concentration in the developer may cause uneven toner concentration along the axial length of the developer carrying member. Specifically, the developer carrying member has a given length in its axial direction, and therefore toner concentration on the developer carrying member at the upstream side of the transport direction may become greater than toner concentration on the developer carrying member at the downstream side.
Such uneven toner concentration on the developer carrying member may result in uneven image concentration on a recording sheet.
In another configuration, used developer may be recovered in the agitation compartment. In such case, the recovered developer in the agitation compartment may be agitated for a length of time that varies depending on a recovery position of developer in the agitation compartment.
For example, toward the downstream side of transport direction in the agitation compartment, recovered developer may be agitated for a shorter time.
In general, an end portion on the downstream side of transport direction of the agitation compartment may communicate with an end portion on the upstream side of transport direction of the supplying compartment.
Accordingly, developer recovered at the downstream side of transport direction of the agitation compartment may be supplied to the supplying compartment within a relatively shorter period of time, which may cause uneven agitation of the recovered developer in the agitation compartment. As a result, the toner may be charged unevenly even if fresh toner is used to refill the developing unit, which may adversely affect image quality and result in uneven and/or decreased image concentration on the recording sheet.
The above-mentioned drawback may occur due to a recovery configuration using a supplying compartment or an agitation compartment to recover developer.
In one conventional example, a developing unit may have a recovery compartment in addition to a supplying compartment and an agitation compartment to cope with such drawback. The recovery compartment may be used to recover used developer in the developing unit.
The supplying compartment may be used to supply developer to a developer carrying member while transporting the developer in the supplying compartment.
The recovery compartment may be used to recover developer and transport the recovered developer in a direction parallel to a developer transport direction of the supplying compartment.
The agitation compartment may be used to agitate excess developer, transported to a downstream side of the transport direction of the supplying compartment, with the recovered developer transported to a downstream side of transport direction of the recovery compartment.
Such supplying compartment, recovery compartment, and agitation compartment may be separated from each other by separation walls.
A first separation wall, set between the agitation compartment and the supplying compartment, may have an opening port through which agitated developer may be transferred from the downstream side of the agitation compartment to the upstream side of the supplying compartment.
The first separation wall, set between the supplying compartment and the agitation compartment, may have another opening port, through which excess developer may be transferred from the downstream side of the supplying compartment to the upstream side of the agitation compartment. Excess developer may mean developer transported in the supplying compartment but not carried up to the developer carrying member.
A second separation wall, set between the recovery compartment and the agitation compartment, may have an opening port, through which recovered developer may be transferred from the downstream side of the recovery compartment to the upstream side of the agitation compartment.
In such configuration, a supply of developer to the developer carrying member and a recovery of developer from the developer carrying member may be conducted separately, by which a toner concentration in developer to be supplied to the developer carrying member may become uniform along an axial length of the developer carrying member. Accordingly, image concentration may become uniform.
Furthermore, in such configuration, an agitation of developer and a recovery of developer may be conducted separately, by which the developer may be agitated effectively. Accordingly, image concentration may become stabilized.
In such developing unit, the agitation compartment may have an agitation screw as an agitating and transporting member. Such agitation screw may agitatingly transport developer in the agitation compartment, and supply developer to an upstream side of the supplying compartment from a downstream side of the agitation compartment.
If the supplying compartment is disposed above the agitation compartment, the agitation screw may need some developer transport force to transport developer from a lower side to an upper side against the force of gravity.
Accordingly, such agitation screw may be manufactured with a given condition based on several factors such as screw pitch and screw rotation speed, so that the agitation screw can provide a developer transport force for transporting developer at the downstream side of the agitation compartment.
Specifically, the agitation screw may have a relatively short screw pitch and a screw vane disposed at an angle closer to 90 degrees, so that developer can be pushed upward from the downstream side of agitation compartment to the upstream side of supplying compartment.
If the agitation screw has a relatively long screw pitch and a screw vane disposed at an angle closer to 0 degrees, the developer may not be effectively pushed upward with the screw vane. Accordingly, the agitation screw may not provide the developer transport force necessary for transporting developer at the downstream side of the agitation compartment. Therefore, the developer may not be effectively transported to the upstream side of supplying compartment.
Further, the agitation screw may have a given screw rotation speed such that the developer may be effectively supplied to the upstream side of supplying compartment from the downstream side of the agitation compartment when the agitation screw has a relatively shorter screw pitch.
In general, a screw manufactured with a same given screw pitch, a same given outer diameter, and a same given rotational speed may be used in the above-mentioned compartments from a viewpoint of manufacturing cost.
However, if an image forming apparatus conducts image forming operations with a higher speed, developer may not be effectively transported to the downstream side of supplying compartment, which may result in insufficient image concentration corresponding to the downstream side of the supplying compartment, which is undesirable.
In such higher speed image forming process, an amount of developer to be supplied to the developer carrying member per unit time may need to be increased. In that case, although the developer may be effectively supplied from the agitation compartment to the upstream side of supplying compartment, an amount of developer to be transported to the downstream side of the supplying compartment by a supplying screw may be insufficient for an amount of developer to be supplied to the developer carrying member.
Therefore, at the downstream side of the supplying compartment, the developer may not be sufficiently supplied to the developer carrying member, and the resultant formed image may not have sufficient image concentration.
Screw rotation speed may be increased to cope with the above-mentioned insufficient supply of the developer at the downstream side of supplying compartment. However, such increased speed may adversely affect durability and generate heat at a shaft bearing of the screw.
In addition, an outer diameter of the screw may be increased to cope with the above-mentioned insufficient supply of the developer at the downstream side of supplying compartment. However, such an increase in the diameter of the screw may hinder efforts to make the developing unit more compact.